A typical disposable electrochemical sensor for blood glucose monitoring includes a substrate film upon which a layer of conductive material is deposited and patterned to form electrodes. Traditionally electrochemical cells, or biosensors, are comprised of three electrodes, a working electrode or sensing electrode, a reference electrode, and a counter electrode or auxiliary electrode. The working electrode is where the reaction of interest occurs at a fixed applied potential versus the reference electrode. The reference electrode functions to maintain a stable electrical potential on the working electrode. The counter electrode allows current to flow between the working electrode and the counter electrode so as not to disturb the reference electrode function. In cases when the system potential is inherently stable or small fluctuations in potential are not a concern, the reference and counter electrodes can be combined into a single reference/counter electrode paired with a working electrode. In some instances electrochemical biosensors use amperometry to quantify specific analyte concentration(s). The working electrode, provides a response proportional to its exposed surface area. During fabrication, the manufacture closely controls the process variation associated with the working electrode area.
Normally the working electrode is formed from two or more elements. One element is a conductive layer that forms the active element facilitating electron transfer to or from an electro-active species which are generated when the sample is applied to the sensor. A second element is a dielectric layer that defines, along with the first element, the actual dimensions of the working electrode that is in contact with the sample fluid. The second element forms a window over a portion of the conductive layer. Variation in either element may result in a variation in the sensor response. The second element or dielectric layer may therefore directly influence the accuracy of the reading.
Some prior art sensors reduce the effects of inaccurately applying the dielectric layer on the final electrode surface area by using a plurality of conductive neck sections in a symmetrical pattern. The window in the dielectric layer may shift slightly, because symmetrically arranged neck sections compensate for the shift. In such sensors, the dielectric layer may be poorly defined but the effect of the poor definition may be minimized because the neck intersecting the dielectric layer edge is very small. However, such sensors require very precise definition of a number of all conductive neck sections, for example, two different neck sections, on a typical sensor. Some of these precisely defined conductive neck sections are not even connected to an external circuit, although it is still necessary to precisely define the neck sections for symmetry, which increases the complexity and costs of fabricating the sensor.
In prior art electrodes the surface areas may be defined by either conductive layer patterning or dielectric layer patterning and registration. There is a need for a means of more accurately defining the sensor's working electrode to simplify the process of forming an accurate biosensor.